This application claims priority to an application entitled xe2x80x9cPHOTODIODE FOR ULTRA HIGH SPEED OPTICAL COMMUNICATION AND FABRICATION METHOD THEREFOR,xe2x80x9d filed in the Korean Industrial Property Office on Jan. 8, 2002 and assigned Serial No. 02-959, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical receiving element, and particularly to a photodiode used in an optical communication system.
2. Description of the Related Art
In the optical communication, an electrical signal is converted into an optical signal at the transmitting end using a light emitting element and then transmitted through a transmission line, such as an optical fiber. The converted optical signal is converted back into an electrical signal at the receiving end using a light receiving element, such as a photodiode. Most widely used photodiodes have a mesa type structure.
FIG. 1 is a cross-sectional view of a conventional photodiode 10 having a mesa type structure. As shown in FIG. 1, the non-doped InGaAs and p-InP are sequentially stacked at one end of an InP substrate 11 by means of a single crystal growing process. An u-InGaAs absorption layer 12 of a mesa type and a p-InP window layer 13 are formed by etching. Thereafter, silicon nitride(SiNx) is stacked on the p-InP window layer 13 so that an insulation layer 14 is formed, and a predetermined portion of the insulation layer 14 is etched so that a part of the p-InP window layer 13 has an opening. A p-type electrode 15 is provided on the open portion of the p-InP window layer 13. Meanwhile, a non-reflection coating is applied to the position corresponding to the p-InP window layer 13 on the other side of the InP substrate 11, so that a light receiving region 17 having a predetermined size and an n-type electrode 16 are formed.
In the method of fabricating a photodiode of a mesa type as described above, the non-doped InGaAs and p-InP layers are stacked through the single crystal growth process. Further, the undesirable spreading of a p-type dopant, such as Zn or Cd, is not necessary.
The conventional photodiode having the mesa type structure, however, has several drawbacks in that the non-doped InGaAs and the p-InP are formed as layers in the form of a mesa type, then exposed to the atmosphere. As such, the non-doped InGaAs and the p-InP materials can be oxidized during a fabrication process. This oxidation may cause a deterioration in the quality of the optical element. In addition, a current leakage may occur on the surface that is mesa-etched, i.e., the surface facing the insulation layer, thereby reducing the life of the optical component. Moreover, the InGaAs film whose energy band gap is small, may have a larger current leakage, thus further deteriorating the reliability of the optical circuit.
Furthermore, in an ultra high-speed optical communication, the capacitance of the optical circuit must be small which can be achieved by reducing the spreading region of the p-InP. The photodiode of a related art, however, makes wire bonding by providing a p-type electrode connected directly to the spreading region of the p-InP. Therefore, it is difficult to reduce the area of the spreading region of the p-InP.
The present invention overcomes the above-described problems, and provides additional advantages, by providing a photodiode used in an ultra high-speed optical communication and its fabrication method capable of improving reliability and lowering the capacitance thereof, by preventing a current leakage and enabling easier subsequent wire bonding.
According to one aspect of the invention, a photodiode for ultra high speed optical communication includes: a substrate; an absorption layer formed on an upper surface of the substrate; a window layer stacked on the absorption layer; an insulation layer stacked on the window layer and having a predetermined hole formed thereon; a spreading region in which a predetermined dopant is spread on the part of the window layer facing the hole; and, an upper electrode connected to the spreading region through the hole and formed in a mushroom shape above the insulation layer.
According to another aspect of the invention, a photodiode for ultra high speed optical communication includes: an InP substrate; an InGaAs absorption layer stacked on one side of the InP substrate; an InP window layer stacked on the InGaAs absorption layer; an insulation layer stacked on the InP window layer and having a predetermined hole formed thereon; a p-InP spreading region in which a p-type dopant is doped on the part of the InP window layer facing the hole; a metal of electrical conductivity electrically connected to the p-InP spreading region through the hole and formed in a mushroom shape above the insulation layer.
According to a further aspect of the invention, a method for fabricating a photodiode for ultra high speed optical communication includes the steps of: preparing a predetermined semiconductor substrate in which a spreading region is included, wherein an absorption layer, a window layer, and an insulation layer are stacked in sequence on the predetermined substrate, and the spreading region is formed by spreading a predetermined dopant on the window layer through an opening formed on the part of the insulation layer; forming a photoresist layer on the insulation layer; forming a hole on the photoresist layer by means of a photoetching method so that the hole is connected to the opening of the insulation layer; forming a metal plated layer connected to the spreading region through the hole formed on the photoresist layer; and, removing the photoresist layer.